Ternary metal fluoride films consisting of fluoride, alkaline earth metal and a fourth period transition metal (e.g., elements 22 to 30 of the periodic table of elements) have been the subject of intense interest because of their electrical, magnetic and optical properties. In addition to basic advantages, such as relatively low refractive indices, low dielectric constants, high levels of optical transparency and excellent mechanical properties, ternary metal fluoride films have drawn interest for a variety of other properties, such as piezoelectric, ferromagnetic or antiferromagnetic, electro-optic, pyroelectric and nonlinear optical properties. An illustration of the known utility of ternary metal fluorides is provided by Bergman, Jr. et al U.S. Pat. No. 3,982,136.
A method of providing metal fluoride layers is described in U.S. Pat. No. 3,475,192-Langley. In this method the substrate is coated with a solution of magnesium fluoride in a polar organic solvent, after which the substrate is heated to a temperature between 100.degree. and 1000.degree. C., preferably between 400.degree. and 800.degree. C. In this method it is necessary to add a film forming vehicle to the solution so as to obtain a layer of an optically good quality. Said film forming vehicle must be fired in the thermal treatment for which a temperature between 400.degree. and 800.degree. C. may haze the magnesium fluoride layer formed.
It has been proposed in U.S. Pat. No. 4,492,721-Joosten et al that magnesium fluoride layers be formed by decomposition of fluorinated organic magnesium compounds such as magnesium trifluoroacetate. However, the preparation of fluorinated metal salts is difficult and therefore the materials for the Joosten et al method are expensive. Furthermore, magnesium trifluoroacetate is toxic and its inhalation may result in severe injury. Further, the method is not believed generally suitable for the preparation of ternary metal fluoride layers.
Paz-Pujalt U.S. Pat. No. 5,051,278 discloses a method of forming a film of metal fluoride comprising combining a non-fluorine-containing metallo-organic compound, solvent and a fluorinating agent to form a casting liquid, forming a coating of the casting liquid on a substrate, and heating the coating and the substrate, wherein the metal of the metallo-oganic compound comprises at least one of lithium, sodium, potassium, magnesium, calcium, strontium, barium, yttrium, europium and other rare earths.
Vacuum vapor deposition processes of forming metal fluoride layers are unattractive for forming ternary metal fluoride layers, since there is no asssurance of uniformly attaining the intended stoichiometry of the metals in the layers. Vacuum vapor deposition also places constraints on coating environments and substrate forms.